More specifically the present invention relates to a compressor device that is at least provided with screw compressor with a compression chamber that is formed by a compression housing, in which a pair of meshed compressor rotors are rotatably mounted, with a drive motor that is provided with a motor chamber formed by a motor housing, in which a motor shaft is rotatably mounted that drives at least one of the aforementioned two compressor rotors, with an inlet to the screw compressor for supplying air, with an outlet from the screw compressor for the discharge of compressed air and which is connected to a pressure vessel via an outlet pipe, with an air outlet from the pressure vessel for supplying the compressed air from the pressure vessel to a consumer, and with a control system for controlling one or more liquid or gas flows in the pneumatic assembly, said control system being provided with an inlet valve at the inlet of the screw compressor and a tap or valve for closing and opening the air outlet of the pressure vessel.
Such compressor devices are already known, which however present a number of disadvantages or which are open to improvement.
Indeed, in the most well known such compressor devices, the screw compressor is driven at a constant speed of rotation by a separate drive motor that is supplied directly from the supply network.
In order to be able to adjust the airflow through the screw compressor, an inlet valve is provided at the inlet of such known screw compressors.
This inlet valve also acts to limit the required torque that has to be delivered by the drive motor when starting up the screw compressor, whereby to limit the required start-up torque the inlet valve is closed during start-up.
On the other hand, in such known compressor devices, after the screw compressor has stopped the compressed air pumped into the pressure vessel by the screw compressor is simply released, again with the intention of limiting the start-up torque as much as possible when restarting the screw compressor.
Starting up with the compression chamber of the screw compressor under pressure would require a very high torque from the drive motor in such compressor devices with a constant speed drive.
If the aforementioned measures were not taken, then the drive motor would not be able to develop enough torque during start-up, or the supply network would not be able to supply the necessary start-up current to develop the high start-up torque.
A considerable disadvantage of these known compressor devices is that a lot of energy is lost through the compressed air already stored in the pressure vessel and in the screw compressor being lost after the screw compressor has stopped.
In another known improved type of compressor device, a solution to the aforementioned disadvantages is partially provided by equipping the screw compressor with a variable speed drive.
In this known type of compressor device the airflow through the screw compressor is adjusted by adapting the speed of rotation of the drive motor, such that no inlet valve is required for this purpose.
Furthermore, when starting up the screw compressor in such a known compressor device, use can also be made of an electronic controller in order to realise a higher starting torque or to limit the starting current drawn from the supply network.
An additional advantage of the application of such an electronic controller is that the compressed air in the pressure vessel does not necessarily have to be released when the screw compressor has stopped, as sufficient torque can be developed when starting up to overcome the pressure in the pressure vessel.
In this way it can be ensured that when the screw compressor is stopped, less energy is lost than with known compressor devices with a constant speed drive.
However, in order to be able to realise this, in the assembly a non-return valve first and foremost has to be provided in the outlet pipe between the outlet of the screw compressor and the pressure vessel, to prevent the compressed air present in the pressure vessel expanding and escaping via the outlet pipe after the screw compressor has stopped, under the influence of the pressure difference between the pressure vessel and the compression chamber of the screw compressor or the ambient pressure.
Moreover, with oil-injected screw compressors an oil separator is normally provided in the pressure vessel, in which oil is separated from the compressed air flow originating from the screw compressor and is guided back to the screw compressor via an oil return pipe affixed between the pressure vessel and the screw compressor.
In such a case when the screw compressor is stopped, the separated oil in the pressure vessel flowing back to the screw compressor must be prevented, as otherwise this would lead to an excess of oil in the screw compressor and could also impede the restart of the screw compressor.
Hence in the known compressor devices of the type discussed above, a non-return valve always has to be provided in the oil return pipe.
A disadvantage of the aforementioned non-return valves is that they give rise to large friction losses.
Moreover, the volume of compressed air in the screw compressor itself is always lost when the screw compressor is stopped, as this compressed air can escape through the inlet of the screw compressor.
Hermetically sealing the inlet by means of an inlet valve with the intention of leaving the screw compressor under pressure when stopped provides no solace here.
In order to be able to drive the compressor rotors, in the known compressor devices generally the motor shaft of the drive motor is directly or indirectly, for example via a drive belt or a gearwheel transmission, coupled to the rotor shaft of one of the compressor rotors.
Hereby the rotor shaft of the compressor concerned must be adequately sealed, which is far from easy.
Indeed, a certain pressure supplied by the screw compressor prevails in the compression housing, which has to be screened off from the compressor sections that are not under this pressure or from the ambient pressure.
For such applications, a “contact seal” is often used.
The application of a sealed inlet valve after the screw compressor has stopped would thus carry a high risk of the occurrence of leaks in the rotor shaft seal.
Moreover, the restart of the screw compressor, when it is under pressure, will be coupled with high friction losses, such that the seal can be easily damaged.
Another disadvantage of the known compressor devices relates to the seal itself of the screw compressor.
The rotor shaft of the compressor rotor concerned turns at very high speeds, such that such a type of seal brings about enormous power losses during the operation of the screw compressor, resulting in a reduced efficiency of the screw compressor.
Moreover, such a “contact seal” is subject to wear, and if it is not carefully installed such a “contact seal” is very sensitive to the occurrence of leaks.
Another aspect of the known compressor devices of the type described above that is open to improvement, is that both the drive motor and the screw compressor have to be provided with lubrication and cooling, that generally consist of separate systems and thus are not attuned to one another, require a number of different types of lubricants and/or coolants, and are thereby complicated or expensive.
In addition, in such known compressor devices with separate cooling systems for the drive motor and compressor rotors, the possibilities for recovering the lost heat stored in the coolants in an optimum way are not fully utilised.